Information transmission systems that use optical fiber as the transmission medium are promising to become a very important and substantial part of the transmission network. Among optical fiber, single mode optical fiber assumes a prominent position because of its potentially extremely high carrying capacity. In order to meet the demanding specifications for optical fiber, and particularly for single mode optical fiber, it is typically necessary to monitor various fiber parameters, including fiber loss and bandwidth. In single mode fiber, two further parameters are of great importance, namely the cut-off wavelength of the first higher order mode and the single mode beam width, herein referred to as the "mode field radius" (MFR). The former is of importance because, inter alia, it determines the minimum operating wavelength of the fiber, and the latter because, inter alia, coupling loss may become large if fiber segments having different MFR are coupled together in multi-segment single mode fiber links. A general reference on optical fibers is Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, editors, Academic Press (1979).
The art knows several techniques for measuring the cut-off wavelength, but they generally fall into one of two classes. Techniques in the first class rely upon the fact that more optical power can be launched into an overfilled fiber core when higher-order modes can propagate than when only the fundamental mode propagates. Thus, a substantial decrease in the power transmitted through a fiber is observed at the cut-off wavelength. Such a measurement can be implemented in at least two ways. First, the loss of a fiber can be measured with and without a "mode-stripping" bend in the fiber, (e.g., an about one-inch diameter loop), and the wavelength above which the results of the two measurements converge can be identified. See, for instance, Y. Katsuyama et al, Electronics Letters, Vol. 12(25), pp. 669-670 (1976). Second, the amount of power launched into a short, e.g., about 1 meter long, fiber, whose loss is negligible, may be measured directly. See, for instance, the paper by P. D. Lazay in Technical Digest, Symposium on Optical Fiber Measurements, Boulder, Colorado, October 1980, pp. 93-95. Both these approaches have shortcomings. In the former technique there exists ambiguity as to whether the observed cut-off is the desired single mode transition, the technique is complicated by fiber loss and mode coupling, and is unsuited for measuring fibers of length greater than about 10 mm. The practice of the latter technique requires a high quality monochromator to yield a smooth launch spectrum. Such an instrument is costly and not well suited to function in a production environment.
The second class of cut-off wavelength measurement techniques uses measurement of the near- or far-field of a fiber. Since the fundamental mode and the higher order modes have substantially different near-field and far-field patterns, the presence of higher order modes can, in principle, be detected. The cut-off wavelength can then be identified as that wavelength above which no higher order mode is present. Measurement of the near-field pattern was proposed, inter alia, by Y. Murakami et al, Applied Optics, Vol. 18(7), pp. 1101-1105 (1979). Since near-field patterns are relatively difficult to measure, more emphasis has apparently been placed upon far-field methods. In particular, the minimum in the far-field pattern has been analyzed. A. R. Tynes et al (Journal of the Optical Society of America, Vol. 69(11), pp. 1587-1596 (1979)), and W. A. Gambling et al (Microwaves, Optics, and Acoustics, Vol. 1(1), pp. 13-17 (1976)) have analyzed the half-power points determined from the measured far-field amplitude. However, the former is not applicable to non-step-index fiber, and the latter yields the theoretical cut-off wavelength, not the effective cut-off.
The art also knows several techniques for determining the MFR of single mode fiber. For instance, one such technique requires focusing a greatly magnified image of the illuminated fiber core into a television vidicon (Y. Murakami et al, op. cit.). Another technique utilizes the sensitivity of splice loss to transverse offset of the coupled fibers to determine the spot size (J. Streckert, Optics Letters, Vol. 5(12), page 505 (1980)). Other techniques require measurement of the far-field radiation field, with computation of either core parameters or the near-field radiation distribution and index profile from the measured pattern. W. A. Gambling et al, Microwaves, Optics, and Acoustics, Vol. 1(1), pp. 13-17 (1976), and K. Hotate and P. Okoshi, Applied Optics, Vol. 18(19), page 3265 (1979), respectively. These techniques also appear to be subject to shortcomings. For instance, the near-field technique requires substantial instrumentation to reach acceptable sensitivity and furthermore, is subject to error due to the marginal linearity of infrared vidicons. The transverse offset technique typically suffers from calibration difficulties and is furthermore subject to end-separation errors that are difficult to control. And lastly, the prior art far-field techniques require a substantial amount of computation, and are therefore time-consuming. A general review of the field of fiber characterization can be found in D. Marcuse, Principles of Optical Fiber Measurements, Academic Press, 1981.
In view of the importance of routine measurement of MFR and cut-off wavelength for, inter alia, fiber manufacturing process control purposes, techniques for measuring these parameters which are suitable for implementation in a production environment, which are fairly rapid, relatively insensitive to operator skill, require only relatively unsophisticated equipment, and which give reliable results even on fibers having nonideal refractive index profiles, e.g., having a central dip and/or a rounded "step" profile, are of considerable practical importance. This application discloses such a method.